Thick film conductors serve as electrical interconnections between resistors, capacitors, inductors, integrated circuits etc. Besides, in multilayer devices and circuits, these conductors interconnect between layers of conductor lines through vias built into the devices. In other words, tape-casted ceramic green sheets for LTCC devices are punched, screened with appropriate conductor lines, stacked and laminated prior to firing. Punching involves mechanical perforation of vias through each layer which are filled with via-fill conductor formulations to interconnect the circuitry between layers. Processing of PTOS circuits are described in a recent invention disclosure U.S. Provisional Patent Application No. 60/703,530, herein incorporated by reference. In other words, interconnect devices are an electronic circuits and surface and buried sub systems of resistors, capacitors, inductors etc that are connected both by electrically and mechanically, “green systems”, electronic systems contain very little or no known toxic materials, is a new goal for environmentally safer to the public-at-large and furthermore the public desired such systems.
Similar to other thick film materials, thick film conductors and via-fills are comprised of an active (conductive) metal and inorganic binders, both of which are in finely divided form and are dispersed in an organic vehicle. The conductive phase is ordinarily gold, palladium, silver, platinum or alloy thereof, the choice of which depends upon the particular performance characteristics which are sought, e.g. resistivity, solderability, solder leach resistance, bondability, adhesion, migration resistance and the like. In multilayer devices, internal conductor lines and via conductors additional performance characteristics which are sought, e.g. minimization of conductor line “sinking” into the top and bottom dielectric layers on firing, resistivity variation on repeated firing, interface connectivity of line conductor to that of via-fill conductor, interface bonding of via-fill conductor to that of surrounding ceramic materials.
Thick film techniques are contrasted with thin film techniques which involve deposition of particles by evaporation or sputtering with or without vacuum. Thick film techniques are well known to those skilled in the art.
In addition to the proper level of conductivity and other properties listed above, there are many secondary properties which must also be present such as, wire bondability, good adhesion to both ceramic and thick films, solderability and compactibility to other thick films, both surface and buried, long-term stability without little or less properties degradation.
As would be expected, one critical variable in the technology of thick film conductors for use in the multilayer interconnect devices is the resistivity variation by interaction with surrounding ceramics. Of particular importance in this regard has been the incorporation of high-melting refractory glasses and glasses have little or no miscibility with the remnant glasses present in the surrounding ceramics. Furthermore, additional incorporation of metal oxide and non-metal oxide binder materials in the composition increases the densification of the conductor composites and/or control the growth of crystalline materials into the conductor composites which would change the resistivity of the composites, a non-desirable result.
Additionally, the second most critical variable in the technology used in the MLI devices is the interface connection of via-fill buried within and the line conductor. Due to the differences in the solids and printing conditions which are needed to fill the vias completely and to connect vias to the line conductors result in conductor-via interface stress development and significant delaminations and microstructure variations at the interface which result in conductivity differences, an unwanted result.
As would be expected, the third most critical variable in the state-of-the-art thick film technology used in the MLI devices is the printing differences needed to fill vias and line conductors, due to the significant viscosity differences of the formulations. Typical via-fill conductor formulation viscosity is in the range of 1500-7000 or more PaS and that of line conductors is typically in the range of 150-300 PaS. These viscosity differences are critical to fill vias and fast printing of line conductor formulations. Lower viscosity via-fill formulations could “flow out of the bottom of the tape” and show “peaking”; and high viscosity formulations need more than one print to fill the vias. Many high viscosity formulations also show “posting” of the vias. On the other hand, line conductor formulation viscosity should be kept low in order to print connective lines without line-break, thinner lines and print the formulation fast enough for better productivity.
The fourth most critical variable is the processing of multilayer interconnect laminate without delamination particularly thicker laminates containing higher number of layers and thicker Green tapes up to 10 mils or higher. These green parts contain large amount of organics. During the heat processing, resins and polymers present will “depolymerize” to monomers, dimers etc. which have higher vapor pressure than the parent polymer, leaves the system at the early stages of the processing. The residual organic components decompose to carbon containing species later in the process.
The temperature profile needed to get rid of all the organics before softening of the glasses present in the tape depends on: quantity and quality of the organics, rate of heating, temperature, firing atmospheric conditions etc. Variations of one or more of the above components result in delaminated of layers, via-to-tape side wall separation, via-to-line conductor separation and finally yield loss.